This invention is directed to heavy-duty vacuum cleaners of the type used in woodshops, machine shops, or for industrial applications, and commonly referred to as shop vacuums or shop vacuum cleaners. The invention is more particularly concerned with a shop vacuum in which there are a series of dust separation stages, with the large majority of the dust being deposited into a drum or barrel, and with the remaining dust that is carried in the vacuum machine air stream being filtered out in a pre-filter and in a final filter, so that the vacuum cleaner machine exhausts clean, filtered air into the ambient.
Shop vacuum cleaners are often employed for picking up dust that has accumulated on the shop floor and on surfaces of equipment, or may also be connected to a dust outlet of a dust-producing tool, i.e., wood working machines, such as sanders, joiners, and the like, or machines that process metals, plastics, or other composites such as concrete or stone. In these shop vacuum devices, a vacuum head, which sits on top of a drum or barrel, a blower that is powered by an electric motor induces a suction to draw a stream of air into the machine. The airstream is then directed into the barrel, where dust collects. The air stream then passes though a bag filter or other filter, and is exhausted to the ambient.
In most cases, the shop vacuum filter does not filter out fine dust, and there is always at least some of the dust that passes out and back into the ambient air. This airborne fine dust can present a health hazard, and in a woodshop environment is a serious quality issue as the airborne dust can contaminate varnish or other wood finish.
The dust that is collected can quickly clog and blind the filter also, which limits air flow and diminishes the efficiency of the shop vacuum. Moreover, filling of the filter material requires that the vacuuming operation be interrupted frequently for cleaning and/or replacing of the filter.
A shop-type vacuum cleaner with a cartridge type final filter to capture fine dust particles has been proposed previously, and an example of such a shop vacuum is described in U.S. Pat. No. 5,069,696. In that case, an externally-mounted filter is located in the exhaust air stream in a housing that is disposed outside the vacuum machine drum or canister. This arrangement exhausts significantly cleaner air back into the ambient, but because the air passes directly from the main collection drum out to the filter housing, the filter accumulates dust quickly and requires frequent cleaning for effective operation.
It has been proposed previously to employ a cyclonic separator in line in a vacuum hose in advance of a shop vacuum cleaner for pre-separating particulate matter, and then connecting the outlet pipe of the cyclonic separator to the inlet of the vacuum cleaner. This arrangement is described, for example, in U.S. Pat. No. 7,282,074. This system can result in removal of about ninety percent, or more, of the dust from the air stream ahead of the vacuum cleaner, so that the vacuum cleaner operates longer and more efficiently in most applications. However, this arrangement requires attaching the cyclone as a separate element in between the shop vacuum cleaner and the dust producing tool or dust pick-up tool.